Erlang中的函数等式和排序

tcomlyy6 于 2022-12-08 发布在 Erlang

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What does it mean to compare functions in Erlang with the operators =:= , == , < , > , =< , >= ?
I was playing around with the interpreter and got these results:

Eshell V5.9.2  (abort with ^G)
1> X = fun() -> {} end.
#Fun<erl_eval.20.82930912>
2> Y = fun() -> {} end.
#Fun<erl_eval.20.82930912>
3> 
3> {X == X, X =:= X}.
{true,true}
4> {X >= X, X =< X}.
{true,true}
5> {X > X, X < X}.
{false,false}
6> 
6> {X == Y, X =:= Y}.
{true,true}
7> {X >= Y, X =< Y}.
{true,true}
8> {X > Y, X < Y}.
{false,false}

This makes sense. It looks like it's comparing the abstract syntax tree of the two functions.
But in this session X and Y are defined the same once again but are different, also now X<Y ?

Eshell V5.9.2  (abort with ^G)
1> X = fun() -> {} end.
#Fun<erl_eval.20.82930912>
2> 
2> {X == X, X =:= X}.
{true,true}
3> {X >= X, X =< X}.
{true,true}
4> {X > X, X < X}.
{false,false}
5> 
5> Y = fun() -> {} end.
#Fun<erl_eval.20.82930912>
6> 
6> {X == Y, X =:= Y}.
{false,false}
7> {X >= Y, X =< Y}.
{false,true}
8> {X > Y, X < Y}.
{false,true}

So it looks like it's not comparing the AST or any sort of unique references. Maybe it is comparing references, just some optimization is happening and X and Y get bound to the same reference? If there is some explanation for this, what happens across different VMs or different nodes?

erlang

来源：https://stackoverflow.com/questions/13077084/function-equality-and-ordering-in-erlang

2条答案

按热度按时间

bwitn5fc1#

shell中2求值之间的差异来自空行6〉。如果您使用函数erlang：fun_info/1查看fun，您将看到在这种情况下，该子句使用不同的数字存储（即2而不是1）。
如果你再次输入Y的定义（没有空行）你会得到一个不好的匹配，如果你之前输入一个空行，它是OK的。
我认为这是使用shell的一个副作用，但是行为在程序中是一致的。当然，〉或〈的意思对于一个有趣的程序来说并不明显，但是==是的。一个好的方面是Erlang项的顺序被定义了，所以可以对任何具有可预测行为的项进行排序：

number < atom < reference < fun < port < pid < tuple < list < bit string

赞(0）回复(0）举报 2022-12-08

h7appiyu2#

This behavior is different in shell and in compiled module. As for == and =:= operators in modules I am pretty sure that these operators return true iff:

Both functions are defined in the same place in code (isn't true in shell!)
Their effective closures are equal according to the respective ( == / =:= ) operator.

For instance with code

test(A, B) ->
  fun(C) -> {A, C} end.

the following shall hold:

> test(1, x) == test(1, y).
true.
> test(1, x) =:= test(1, y).
true.
> test(1, x) == test(1.0, y).
true.
> test(1, x) =:= test(1.0, y).
false.

Note that B does not belong to the closure of inner function therefore not influencing the comparison result.

赞(0）回复(0）举报 2022-12-08

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Erlang中的函数等式和排序

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